There has been substantial interest in the preparation and characterisation of compound semiconductors comprising of particles with dimensions in the order of 2-100 nm, often referred to as quantum dots and nanocrystals mainly because of their optical, electronic or chemical properties. These interests have occurred mainly due to their size-tunable electronic, optical and chemical properties and the need for the further miniaturization of both optical and electronic devices that now range from commercial applications as diverse as biological labelling, solar cells, catalysis, biological imaging, light-emitting diodes amongst many new and emerging applications.
Although some earlier examples appear in the literature, recently methods have been developed from reproducible “bottom up” techniques, whereby particles are prepared atom-by-atom, i.e. from molecules to clusters to particles using “wet” chemical procedures. Rather from “top down” techniques involving the milling of solids to finer and finer powders.
To-date the most studied and prepared of nano-semiconductor materials have been the chalcogenides II-VI materials namely ZnS, ZnSe, CdS, CdSe, CdTe; most noticeably CdSe due to its tunability over the visible region of the spectrum. Semiconductor nanoparticles are of academic and commercial interest due to their differing and unique properties from those of the same material, but in the macro crystalline bulk form. Two fundamental factors, both related to the size of the individual nanoparticle, are responsible for these unique properties.
The first is the large surface to volume ratio; as a particle becomes smaller, the ratio of the number of surface atoms to those in the interior increases. This leads to the surface properties playing an important role in the overall properties of the material.
The second factor is that, with semiconductor nanoparticles, there is a change in the electronic properties of the material with size, moreover, the band gap gradually becoming larger because of quantum confinement effects as the size of the particles decreases. This effect is a consequence of the confinement of an ‘electron in a box’ giving rise to discrete energy levels similar to those observed in atoms and molecules, rather than a continuous band as in the corresponding bulk semiconductor material. For a semiconductor nanoparticle, because of the physical parameters, the “electron and hole”, produced by the absorption of electromagnetic radiation, a photon, with energy greater then the first excitonic transition, are closer together than in the corresponding macrocrystalline material, so that the Coulombic interaction cannot be neglected. This leads to a narrow bandwidth emission, which is dependent upon the particle size and composition. Thus, quantum dots have higher kinetic energy than the corresponding macrocrystalline material and consequently the first excitonic transition (band gap) increases in energy with decreasing particle diameter.
The coordination about the final inorganic surface atoms in any core, core-shell or core-multi shell nanoparticles is incomplete, with highly reactive “dangling bonds” on the surface, which can lead to particle agglomeration. This problem is overcome by passivating (capping) the “bare” surface atoms with protecting organic groups. The capping or passivating of particles not only prevents particle agglomeration from occurring, it also protects the particle from its surrounding chemical environment, along with providing electronic stabilization (passivation) to the particles in the case of core material.
The capping agent usually takes the form of a Lewis base compound covalently bound to surface metal atoms of the outer most inorganic layer of the particle, but more recently, so as to incorporate the particle into a composite, an organic system or biological system can take the form of, an organic polymer forming a sheaf around the particle with chemical functional groups for further chemical synthesis, or an organic group bonded directly to the surface of the particle with chemical functional groups for further chemical synthesis.
Single core nanoparticles, which consist of a single semiconductor material along with an outer organic passivating layer, tend to have relatively low quantum efficiencies due to electron-hole recombination occurring at defects and dangling bonds situated on the nanoparticle surface which lead to non-radiative electron-hole recombinations.
One method to eliminate defects and dangling bonds is to grow a second material, having a wider band-gap and small lattice mismatch with the core material, epitaxially on the surface of the core particle, (e.g. another II-VI material) to produce a “core-shell particle”. Core-shell particles separate any carriers confined in the core from surface states that would otherwise act as non-radiative recombination centres. One example is ZnS grown on the surface of CdSe cores. The shell is generally a material with a wider bandgap then the core material and with little lattice mismatch to that of the core material, so that the interface between the two materials has as little lattice strain as possible. Excessive strain can further result in defects and non-radiative electron-hole recombination resulting in low quantum efficiencies.
Quantum Dot-Quantum Wells
Another approach which can further enhance the efficiencies of semiconductor nanoparticles is to prepare a core-multi shell structure where the “electron-hole” pair are completely confined to a single shell such as a quantum dot-quantum well structure. Here, the core is of a wide bandgap material, followed by a thin shell of narrower bandgap material, and capped with a further wide bandgap layer, such as CdS/HgS/CdS grown using a substitution of Hg for Cd on the surface of the core nanocrystal to deposit just a few monolayer of HgS. The resulting structures exhibited clear confinement of photoexcited carriers in the HgS. Other known Quantum dot quantum well (QDQW) structures include—ZnS/CdSe/ZnS, CdS/CdSe/CdS and ZnS/CdS/ZnS.
Colloidally grown QD-QW nanoparticles are relatively new. The first and hence most studied systems were of CdS/HgS/CdS grown by the substitution of cadmium for mercury on the core surface to deposit one monolayer of HgS. A wet chemical synthetic method for the preparation of spherical CdS/HgS/CdS quantum wells was presented with a study of their unique optical properties. The CdS/HgS/CdS particles emitted a red band-edge emission originating from the HgS layer. Little et al. have grown ZnS/CdS/ZnS QDQWs using a similar growth technique to that of Eychmüller to show that these structure can be made despite the large lattice mismatch (12%) between the two materials, ZnS and CdS. Daniels et al produced a series of structures that include ZnS/CdSe/ZnS, ZnS/CdS/CdSe/ZnS, ZnS/CdSe/CdS/ZnS, ZnS/CdS/CdSe/CdS/ZnS. The aim of this work was to grow strained nanocrystalline heterostructures and to correlate their optical properties with modelling that suggested that there is relocation of the carriers (hole/electron) from confinement in the ZnS core to the CdSe shell. CdS/CdSe/CdS QDQW's, have also been produced by Peng et al. although this structure is promising, the small CdS band gap may not be sufficient to prevent the escape of electrons to the surface.5,6,7,8 
Although there are now a number of methods for preparing core-shell quantum dots, where it has been shown and reported for the reaction solutions containing the quantum dots, core-shell quantum dots can have quantum yields as high as 90%. However, it is well known that once one tries to manipulate the freshly made solutions of core-shell quantum dots such as isolating the particles as dry powders, upon re-dissolving the particles quantum yields can be substantially lower (sometimes as low as 1-5%).
According to a first aspect of the present invention there is provided a method for producing a nanoparticle comprised of a core comprising a core semiconductor material, a first layer comprising a first semiconductor material provided on said core and a second layer comprising a second semiconductor material provided on said first layer, said core semiconductor material being different to said first semiconductor material and said first semiconductor material being different to said second semiconductor material, wherein the method comprises effecting conversion of a nanoparticle core precursor composition to the material of the nanoparticle core, depositing said first layer on said core and depositing said second layer on said first layer, said core precursor composition comprising a first precursor species containing a first ion to be incorporated into the growing nanoparticle core and a separate second precursor species containing a second ion to be incorporated into the growing nanoparticle core, said conversion being effected in the presence of a molecular cluster compound under conditions permitting seeding and growth of the nanoparticle core.
This aspect of the present invention relates to a method of producing core/multishell nanoparticles of any desirable form and allows ready production of a monodisperse population of such particles which are consequently of a high purity. It is envisaged that the invention is suitable for producing nanoparticles of any particular size, shape or chemical composition. A nanoparticle may have a size falling within the range 2-100 nm. A sub-class of nanoparticles of particular interest is that relating to compound semiconductor particles, also known as quantum dots or nanocrystals.
The current invention concerns the large scale synthesis of nanoparticles by the reaction whereby a seeding molecular cluster is placed in a dispersing medium or solvent (coordinating or otherwise) in the presence of other precursors to initiate particle growth. The invention uses a seeding molecular cluster as a template to initiate particle growth from other precursors present within the reaction medium. The molecular cluster to be used as the seeding agent can either be prefabricated or produced in situ prior to acting as a seeding agent.
Although manipulation of freshly made solutions of core-shell quantum dots can substantially lower the particles' quantum yields, by using a core-multishell architecture rather than known core-shell structures, more stable nanoparticles (to both chemical environment and photo effects) can be produced. It will be appreciated that while the first aspect of the present invention defines a method for producing nanoparticles having a core, and first and second layers, the method forming the first aspect of the present invention may be used to provide nanoparticles comprising any desirable number of additional layers (e.g. third, fourth and fifth layers provides on the second, third and fourth layers respectively) of pure or doped semiconductor materials, materials having a ternary or quaternary structure, alloyed materials, metallic materials or non-metallic materials. The invention addresses a number of problems, which include the difficulty of producing high efficiency blue emitting dots.
The nanoparticle core, first and second semiconductor materials may each possess any desirable number of ions of any desirable element from the periodic table. Each of the core, first and second semiconductor material is preferably separately selected from the group consisting of a semiconductor material incorporating ions from groups 12 and 15 of the periodic table, a semiconductor material incorporating ions from groups 13 and 15 of the periodic table, a semiconductor material incorporating ions from groups 12 and 16 of the periodic table, a semiconductor material incorporating ions from groups 14 and 16 of the periodic table and a semiconductor material incorporating ions from groups 11, 13 and 16 of the periodic table.
Thus, while at least one of the core, first and second semiconductor materials may incorporate ions from groups 12 and 15 of the periodic table, the material(s) used in these layers may include ions of one or more further elements, for example, more than one element from group 12 and/or group 15 of the periodic table and/or ions from at least one different group of the periodic table. A preferred core/multishell architecture comprises at least one layer incorporating two different types of group 12 ions (e.g. Cd and Zn, or Cd and Hg) and group 16 ions (e.g. S, Se or Te).
In the nanoparticle of the present invention where at least one of the core, first and second semiconductor materials is selected from the group consisting of a semiconductor material incorporating ions from groups 12 and 15 of the periodic table (a ‘II-V’ semiconductor material), a semiconductor material incorporating ions from groups 14 and 16 of the periodic table (a ‘IV-VI’ semiconductor material) and a semiconductor material incorporating ions from groups 11, 13 and 16 of the periodic table (a ‘I-III-VI’ semiconductor material), any other core, first or second layers in a particular nanoparticle may comprise a II-V, IV-VI or I-II-VI material. For example, where a nanoparticle in accordance with the present invention has a core comprising a II-V semiconductor material, the nanoparticle may possess a first layer comprising any appropriate semiconductor material for example a different II-V material (i.e. a II-V material in which the II ions are ions of a different element of group 12 compared to the II ions in the core material and/or the V ions are ions of a different element compared to the group 15 ions in the core material), or a IV-VI or I-III-VI semiconductor material. Furthermore, if the nanoparticle in accordance with the present invention possess a second layer comprising a I-III-VI semiconductor material, it may possess a first layer comprising any suitable semiconductor material including a different I-III-VI semiconductor material, or a II-V or IV-VI material. It will be appreciated that when choosing suitable semiconductor materials to place next to one another in a particular nanoparticle (e.g. when choosing a suitable first layer material for deposition on a core, or a suitable second layer material for deposition on a first layer) consideration should be given to matching the crystal phase and lattice constants of the materials as closely as possible.
The method forming the first aspect of the present invention may be used to produce a nanoparticle comprised of a core comprising a core semiconductor material, a first layer comprising a first semiconductor material provided on said core and a second layer comprising a second semiconductor material provided on said first layer, said core semiconductor material being different to said first semiconductor material and said first semiconductor material being different to said second semiconductor material, wherein                a) at least two of the core, first shell and second shell materials incorporate ions from groups 12 and 15 of the periodic table, groups 14 and 16 of the periodic table, or groups 11, 13 and 16 of the periodic table;        b) the second shell material incorporates ions of at least two different elements from group 12 of the periodic table and ions from group 16 of the periodic table;        c) at least one of the core, first and second semiconductor materials incorporates ions from groups 11, 13 and 16 of the periodic table and at least one other of the core, first and second semiconductor materials is a semiconductor material not incorporating ions from groups 11, 13 and 16 of the periodic table.        
Preferably in set a) the other of the core, first and second semiconductor materials incorporates ions from the group consisting groups 12 and 15 of the periodic table, groups 13 and 15 of the periodic table, groups 12 and 16 of the periodic table, groups 14 and 16 of the periodic table, and groups 11, 13 and 16 of the periodic table.
It is preferred that in set b) said second semiconductor material has the formula MxN1-xE, where M and N are the group 12 ions, E is the group 16 ion, and 0<x<1. It is preferred that 0.1<x<0.9, more preferably 0.2<x<0.8, and most preferably 0.4<x<0.6. Particularly preferred nanoparticles have the structure ZnS/CdSe/CdxZn1-xS/CdxZn1-xS/CdSe/ZnS or CdxZn1-xS/CdSe/CdxZn1-xS.
In a preferred embodiment of set c) said at least one other of the core, first and second semiconductor materials not incorporating ions from groups 11, 13 and 16 of the periodic table incorporates ions from the group consisting of groups 12 and 15 of the periodic table, groups 13 and 15 of the periodic table, groups 12 and 16 of the periodic table, and groups 14 and 16 of the periodic table.
Preferably the nanoparticle formed using the method according to the first aspect of the present invention further comprises a third layer of a third semiconductor material provided on said second layer. The nanoparticle may optionally comprise still further layers of semiconductor material, such as fourth, fifth, and sixth layers.
It is preferred that the third semiconductor material is selected from the group consisting of a semiconductor material incorporating ions from groups 12 and 15 of the periodic table, a semiconductor material incorporating ions from groups 13 and 15 of the periodic table, a semiconductor material incorporating ions from groups 12 and 16 of the periodic table, a semiconductor material incorporating ions from groups 14 and 16 of the periodic table and a semiconductor material incorporating ions from groups 11, 13 and 16 of the periodic table.
Preferably the group 12 ions are selected from the group consisting of zinc ions, cadmium ions and mercury ions. The group 15 ions are preferably selected from the group consisting of nitride ions, phosphide ions, arsenide ions, and antimonide ions. It is preferred that the group 14 ions are selected from the group consisting of lead ions, tin ions and germanium ions. Preferably the group 16 ions are selected from the group consisting of sulfide ions, selenide ions and telluride ions. The group 11 ions are preferably selected from the group consisting of copper ions, silver ions and gold ions. In a preferred embodiment the group 13 ions are selected from the group consisting of aluminium ions, indium ions and gallium ions.
The core, first and second semiconductor materials may include ions in an approximate 1:1 ratio (i.e. having a stoichiometry of 1:1). For example, the nanoparticle ZnS/CdTe/ZnS contains a first layer of CdTe in which the ratio of cadmium to telluride ions is approximately 1:1. The semiconductor materials may possess different stroichiometries, for example the nanoparticle ZnS/CuInS2/ZnS contains a first layer of CuInS2 in which the ratio of copper to indium ions is approximately 1:1 but the ratio of copper to sulfide ions is 1:2 and the ratio of indium to sulfide ions is 1:2. Moreover, the semiconductor materials may possess non-empirical stoichiometries. For example, the nanoparticle ZnS/CuInS2/CdxZn1-xS incorporates a second layer of CdxZn1-xS where 0<x<1. The notation MxN1-xE is used herein to denote a mixture of ions M, N and E (e.g. M=Cd, N=Zn, E=S) contained in a semiconductor material. Where the notation MxN1-xE is used it is preferred that 0<x<1, preferably 0.1<x<0.9, more preferably 0.2<x<0.8, and most preferably 0.4<x<0.6.
The temperature of the dispersing medium containing the growing nanoparticles may be increased at any appropriate rate depending upon the nature of the nanoparticle core precursor composition and the molecular cluster compound being used. Preferably the temperature of the dispersing medium is increased at a rate in the range 0.05° C./min to 1° C./min, more preferably at a rate in the range 0.1° C./min to 1° C./min, and most preferably the temperature of the dispersing medium containing the growing nanoparticles is increased at a rate of approximately 0.2° C./min.
Any suitable molar ratio of the molecular cluster compound to first and second nanoparticle core precursors may be used depending upon the structure, size and composition of the nanoparticles being formed, as well as the nature and concentration of the other reagents, such as the nanoparticle core precursor(s), capping agent, size-directing compound and solvent. It has been found that particularly useful ratios of the number of moles of cluster compound compared to the total number of moles of the first and second precursor species preferably lie in the range 0.0001-0.1 (no. moles of cluster compound): 1 (total no. moles of first and second precursor species), more preferably 0.001-0.1:1, yet more preferably 0.001-0.060:1. Further preferred ratios of the number of moles of cluster compound compared to the total number of moles of the first and second precursor species lie in the range 0.002-0.030:1, and more preferably 0.003-0.020:1. In particular, it is preferred that the ratio of the number of moles of cluster compound compared to the total number of moles of the first and second precursor species lies in the range 0.0035-0.0045:1.
It is envisaged that any suitable molar ratio of the first precursor species compared to the second precursor species may be used. For example, the molar ratio of the first precursor species compared to the second precursor species may lie in the range 100-1 (first precursor species): 1 (second precursor species), more preferably 50-1:1. Further preferred ranges of the molar ratio of the first precursor species compared to the second precursor species lie in the range 40-5:1, more preferably 30-10:1. In certain applications it is preferred that approximately equal molar amounts of the first and second precursor species are used in the method of the invention. The molar ratio of the first precursor species compared to the second precursor species preferably lies in the range 0.1-1.2:1, more preferably, 0.9-1.1:1, and most preferably 1:1. In other applications, it may be appropriate to use approximately twice the number of moles of one precursor species compared to the other precursor species. Thus the molar ratio of the first precursor species compared to the second precursor species may lie in the range 0.4-0.6:1, more preferably the molar ratio of the first precursor species compared to the second precursor species is 0.5:1. It is to be understood that the above precursor molar ratios may be reversed such that they relate to the molar ratio of the second precursor species compared to the first precursor species. Accordingly, the molar ratio of the second precursor species compared to the first precursor species may lie in the range 100-1 (second precursor species): 1 (first precursor species), more preferably 50-1:1, 40-5:1, or 30-10:1. Furthermore, the molar ratio of the second precursor species compared to the first precursor species may lie in the range 0.1-1.2:1, 0.9-1.1:1, 0.4-0.6:1, or may be 0.5:1.
In a preferred embodiment of the first aspect of the present invention the molecular cluster compound and core precursor composition are dispersed in a suitable dispersing medium at a first temperature and the temperature of the dispersing medium containing the cluster compound and core precursor composition is then increased to a second temperature which is sufficient to initiate seeding and growth of the nanoparticle cores on the molecular clusters of said compound.
Preferably the first temperature is in the range 50° C. to 100° C., more preferably in the range 70° C. to 80° C., and most preferably the first temperature is approximately 75° C.
The second temperature may be in the range 120° C. to 280° C. More preferably the second temperature is in the range 150° C. to 250° C., and most preferably the second temperature is approximately 200° C.
The temperature of the dispersing medium containing the cluster compound and core precursor composition may be increased from the first temperature to the second temperature over a time period of up to 48 hours, more preferably up to 24 hours, yet more preferably 1 hour to 24 hours, and most preferably over a time period in the range 1 hour to 8 hours.
In a further preferred embodiment of the first aspect of the present invention the method comprises                a. dispersing the molecular cluster compound and an initial portion of the nanoparticle core precursor composition which is less than the total amount of the nanoparticle core precursor composition to be used to produce said nanoparticle cores in a suitable dispersing medium at a first temperature;        b. increasing the temperature of the dispersing medium containing the cluster compound and core precursor composition to a second temperature which is sufficient to initiate seeding and growth of the nanoparticle cores on the molecular clusters of said molecular cluster compound; and        c. adding one or more further portions of the nanoparticle core precursor composition to the dispersing medium containing the growing nanoparticle cores,wherein the temperature of the dispersing medium containing the growing nanoparticle cores is increased before, during and/or after the addition of the or each further portion of the nanoparticle core precursor composition.        
In this preferred embodiment less than the total amount of precursor to be used to produce the nanoparticle cores is present in the dispersing medium with the cluster compound prior to the initiation of nanoparticle growth and then as the reaction proceeds and the temperature is increased, additional amounts of core precursors are periodically added to the reaction mixture in the dispersing medium. Preferably the additional core precursors are added either dropwise as a solution or as a solid.
The temperature of the dispersing medium containing the growing nanoparticle cores may be increased at any appropriate rate depending upon the nature of the nanoparticle core precursor composition and the molecular cluster compound being used. Preferably the temperature of the dispersing medium is increased at a rate in the range 0.05° C./min to 1° C./min, more preferably at a rate in the range 0.1° C./min to 1° C./min, and most preferably the temperature of the dispersing medium containing the growing nanoparticle cores is increased at a rate of approximately 0.2° C./min.
While the first and second temperatures of the dispersing medium may take any suitable value, in a preferred embodiment of the present invention said first temperature is in the range 15° C. to 60° C. Said second temperature may be in the range 90° C. to 150° C.
It is preferred that the or each further portion of the nanoparticle core precursor composition is added dropwise to the dispersing medium containing the growing nanoparticle cores.
The or each further portion of the nanoparticle core precursor composition may be added to the dispersing medium containing the growing nanoparticle cores at any desirable rate. It is preferred that the core precursor composition is added to the dispersing medium at a rate in the range 0.1 ml/min to 20 ml/min per litre of dispersing medium, more preferably at a rate in the range 1 ml/min to 15 ml/min per litre of dispersing medium, and most preferably at a rate of around 5 ml/min per litre of dispersing medium.
Preferably said initial portion of the nanoparticle core precursor composition is less than or equal to approximately 90% of the total amount of the nanoparticle core precursor composition to be used to produce said nanoparticle cores. Said initial portion of the nanoparticle core precursor composition may be less than or equal to approximately 10% of the total amount of the nanoparticle core precursor composition to be used to produce said nanoparticle cores.
In a preferred embodiment where one further portion of the nanoparticle core precursor composition is added to the dispersing medium containing the growing nanoparticle cores said one further portion is less than or equal to approximately 90% of the total amount of the nanoparticle core precursor composition to be used to produce said nanoparticle cores.
In a further preferred embodiment where more than one further portion of the nanoparticle core precursor composition is added to the dispersing medium containing the growing nanoparticle cores, each of said further portions is less than or equal to approximately 45% of the total amount of the nanoparticle core precursor composition to be used to produce said nanoparticle cores. Each of said further portions may be less than or equal to approximately 10% of the total amount of the nanoparticle core precursor composition to be used to produce said nanoparticle cores.
It is preferred that formation of said molecular cluster compound is effected in situ in said dispersing medium prior to dispersing the molecular cluster compound and the initial portion of the nanoparticle core precursor composition in said dispersing medium.
In a preferred embodiment of the present invention said process is subject to the proviso that the nanoparticle core precursor composition does not contain Cd(CH3CO2)2. A further preferred embodiment provides that said process is subject to the proviso that the nanoparticle core precursor composition does not contain TOPSe. Said process may be subject to the proviso that the nanoparticle core precursor composition does not contain Cd(CH3CO2)2 and TOPSe. In a still further preferred embodiment said process is subject to the proviso that the temperature of the dispersing medium containing the growing nanoparticle cores is increased at a rate which is other than 50° C. over a period of 24 hours.
The conversion of the core precursor to the material of the nanoparticles can be conducted in any suitable dispersing medium or solvent. In the method of the present invention it is important to maintain the integrity of the molecules of the cluster compound. Consequently, when the cluster compound and nanoparticle core precursor are introduced in to the dispersing medium or solvent the temperature of the medium/solvent must be sufficiently high to ensure satisfactory dissolution and mixing of the cluster compound it is not necessary that the present compounds are fully dissolved but desirable. It is most preferred that the temperature of the dispersing medium containing the cluster and precursors should not be so high as to disrupt the integrity of the cluster compound molecules. Once the cluster compound and core precursor composition are sufficiently well dissolved in the solvent the temperature of the solution thus formed is raised to a temperature, or range of temperatures, which is/are sufficiently high to initiate nanoparticle core growth but not so high as to damage the integrity of the cluster compound molecules. As the temperature is increased further quantities of core precursor are added to the reaction, preferably in a dropwise manner or as a solid. The temperature of the solution can then be maintained at this temperature or within this temperature range for as long as required to form nanoparticle cores possessing the desired properties.
A wide range of appropriate dispersing media/solvents are available. The particular dispersing medium used is usually at least partly dependent upon the nature of the reacting species, i.e. nanoparticle core precursor and/or cluster compound, and/or the type of nanoparticles which are to be formed. Preferred dispersing media include Lewis base type coordinating solvents, such as a phosphine (e.g. TOP), a phosphine oxide (e.g. TOPO) or an amine (e.g. HDA), or non-coordinating organic solvents, e.g. alkanes and alkenes (e.g. octadecene). If a non-coordinating solvent is used then it will usually be used in the presence of a further coordinating agent to act as a capping agent for the following reason.
If the nanoparticles being formed are intended to function as quantum dots it is important that the surface atoms which are not fully coordinated “dangling bonds” are capped to minimise non-radiative electron-hole recombinations and inhibit particle agglomeration which can lower quantum efficiencies or form aggregates of nanoparticles. A number of different coordinating solvents are known which can also act as capping or passivating agents, e.g. TOP, TOPO, HDA or long chain organic acids such as myristic acid. If a solvent is chosen which cannot act as a capping agent then any desirable capping agent can be added to the reaction mixture during nanoparticle growth. Such capping agents are typically Lewis bases but a wide range of other agents are available, such as oleic acid and organic polymers which form protective sheaths around the nanoparticles.
The first aspect of the present invention comprises of a method to produce nanoparticle materials using molecular clusters, whereby the clusters are defined identical molecular entities, as compared to ensembles of small nanoparticles, which inherently lack the anonymous nature of molecular clusters. The invention consists of the use of molecular clusters as templates to seed the growth of nanoparticle cores, whereby other molecular sources, i.e. the precursor compounds, or “molecular feedstocks” are consumed to facilitate particle growth. The molecular sources (i.e. core precursor composition) are periodically added to the reaction solution so as to keep the concentration of free ions to a minimum but also maintain a concentration of free ions to inhibit Oswards ripening from occurring and defocusing of nanoparticle size range from occurring.
A further preferred embodiment of the first aspect of the present invention provides that the method comprises:                i. monitoring the average size of the nanoparticle cores being grown; and        ii. terminating nanoparticle core growth when the average nanoparticle size reaches a predetermined value.        
It is preferred that the average size of the nanoparticle cores being grown is monitored by UV-visible absorption spectroscopy. The average size of the nanoparticle cores being grown may be monitored by photoluminescence spectroscopy. Preferably nanoparticle core growth is terminated by reducing the temperature of the dispersing medium from the second temperature to a third temperature.
Conveniently the method may comprise forming a precipitate of the nanoparticle core material by the addition of a precipitating reagent, which is preferably selected from the group consisting of ethanol and acetone.
Preferably conversion of the core precursor composition to the nanoparticle core is effected in a reaction medium and said nanoparticle core is isolated from said reaction medium prior to deposition of the first layer.
It is preferable that deposition of said first layer comprises effecting conversion of a first semiconductor material precursor composition to said first semiconductor material. The first semiconductor material precursor composition preferably comprises third and fourth precursor species containing the ions to be incorporated into the growing first layer of the nanoparticle. The third and fourth precursor species may be separate entities contained in said first semiconductor material precursor composition, or the third and fourth precursor species may be combined in a single entity contained in the first semiconductor material precursor composition.
Preferably deposition of said second layer comprises effecting conversion of a second semiconductor material precursor composition to said second semiconductor material. The second semiconductor material precursor composition preferably comprises fifth and sixth precursor species containing the ions to be incorporated into the growing second layer of the nanoparticle. It is preferred that the fifth and sixth precursor species are separate entities contained in said second semiconductor material precursor composition, alternatively the fifth and sixth precursor species may be combined in a single entity contained in said second semiconductor material precursor composition.
A second aspect of the present invention provides a nanoparticle produced according to a method in accordance with the first aspect of the present invention.
A third aspect of the present invention provides a nanoparticle comprised of a core comprising a core semiconductor material, a first layer comprising a first semiconductor material provided on said core and a second layer comprising a second semiconductor material provided on said first layer, said core semiconductor material being different to said first semiconductor material and said first semiconductor material being different to said second semiconductor material, wherein                a) at least two of the core, first shell and second shell materials incorporate ions from groups 12 and 15 of the periodic table, groups 14 and 16 of the periodic table, or groups 11, 13 and 16 of the periodic table;        b) the second shell material incorporates ions of at least two different elements from group 12 of the periodic table and ions from group 16 of the periodic table;        c) at least one of the core, first and second semiconductor materials incorporates ions from groups 11, 13 and 16 of the periodic table and at least one other of the core, first and second semiconductor materials is a semiconductor material not incorporating ions from groups 11, 13 and 16 of the periodic table.        
Preferably in set a) the other of the core, first and second semiconductor materials incorporates ions from the group consisting groups 12 and 15 of the periodic table, groups 13 and 15 of the periodic table, groups 12 and 16 of the periodic table, groups 14 and 16 of the periodic table, and groups 11, 13 and 16 of the periodic table.
It is preferred that in set b) said second semiconductor material has the formula MxN1-xE, where M and N are the group 12 ions, E is the group 16 ion, and 0<x<1. It is preferred that 0.1<x<0.9, more preferably 0.2<x<0.8, and most preferably 0.4<x<0.6. Particularly preferred nanoparticles have the structure ZnS/CdSe/CdxZn1-xS, CdxZn1-xS/CdSe/ZnS or CdxZn1-xS/CdSe/CdxZn1-xS.
In a preferred embodiment of set c) said at least one other of the core, first and second semiconductor materials not incorporating ions from groups 11, 13 and 16 of the periodic table incorporates ions from the group consisting of groups 12 and 15 of the periodic table, groups 13 and 15 of the periodic table, groups 12 and 16 of the periodic table, and groups 14 and 16 of the periodic table.
Preferably the nanoparticle further comprises a third layer of a third semiconductor material provided on said second layer. The nanoparticle may optionally comprise still further layers of semiconductor material, such as fourth, fifth, and sixth layers.
Regarding the third aspect of the present invention it is preferred that the third semiconductor material is selected from the group consisting of a semiconductor material incorporating ions from groups 12 and 15 of the periodic table, a semiconductor material incorporating ions from groups 13 and 15 of the periodic table, a semiconductor material incorporating ions from groups 12 and 16 of the periodic table, a semiconductor material incorporating ions from groups 14 and 16 of the periodic table and a semiconductor material incorporating ions from groups 11, 13 and 16 of the periodic table.
Preferably the group 12 ions are selected from the group consisting of zinc ions, cadmium ions and mercury ions.
The group 15 ions are preferably selected from the group consisting of nitride ions, phosphide ions, arsenide ions, and antimonide ions.
It is preferred that the group 14 ions are selected from the group consisting of lead ions, tin ions and germanium ions.
Preferably the group 16 ions are selected from the group consisting of sulfide ions, selenide ions and telluride ions.
The group 11 ions are preferably selected from the group consisting of copper ions, silver ions and gold ions.
In a preferred embodiment the group 13 ions are selected from the group consisting of aluminium ions, indium ions and gallium ions.
The current invention describes the design and preparation methods of a number of unique quantum dot-quantum wells nanoparticles including, ZnS/CuInS2/ZnS, ZnS/CuInS2/CdxZn1-xS, CdxZn1-xS/CuInS2/CdxZn1-xS, ZnS/CuGaS2/ZnS, ZnS/CuGaS2/CdxZn1-xS, CdxZn1-xS/CuGaS2/CdxZn1-xS, ZnS/CuInSe2/ZnS, ZnS/CuInSe2/CdxZn1-xS, CdxZn1-xS/CuInSe2/CdxZn1-xS ZnS/CuGaSe2/ZnS, ZnS/CuGaSe2/CdxZn1-xS and CdxZn1-xS/CuGaSe2/CdxZn1-xS, where 0<x<1.
A fourth aspect of the present invention provides a method for producing a nanoparticle according to the third aspect of the present invention, wherein the method comprises effecting conversion of a nanoparticle core precursor composition to the material of the nanoparticle core, depositing said first layer on said core and depositing said second layer on said first layer.
It will be evident to the skilled person how the method forming the fourth aspect of the present invention may be put in to effect by routine modification to the experimental details disclosed herein and involving no undue experimentation for the preparation of core/multishell nanoparticles in accordance with the third aspect of the present invention.
Preferably said nanoparticle core precursor composition comprises first and second core precursor species containing the ions to be incorporated into the growing nanoparticle core. It is preferred that the first and second core precursor species are separate entities contained in the core precursor composition, and the conversion is effected in the presence of a molecular cluster compound under conditions permitting seeding and growth of the nanoparticle core.
The first and second core precursor species may be combined in a single entity contained in the core precursor composition.
Preferably conversion of the core precursor composition to the nanoparticle core is effected in a reaction medium and said nanoparticle core is isolated from said reaction medium prior to deposition of the first layer.
In a preferred embodiment of the fourth aspect of the present invention deposition of the first layer comprises effecting conversion of a first semiconductor material precursor composition to said first semiconductor material.
Preferably the first semiconductor material precursor composition comprises third and fourth precursor species containing the ions to be incorporated into the growing first layer of the nanoparticle. The third and fourth precursor species may be separate entities contained in the first semiconductor material precursor composition (i.e. the precursor species may be provided by a ‘multi source’ or ‘dual source’ precursor composition). Alternatively or additionally the third and fourth precursor species may be combined in a single entity contained in the first semiconductor material precursor composition (i.e. the precursor composition may contain a ‘single source’ precursor comprising both the third and fourth ions to be incorporated in to the first layer).
Preferably deposition of the second layer comprises effecting conversion of a second semiconductor material precursor composition to said second semiconductor material.
Preferably the second semiconductor material precursor composition comprises fifth and sixth precursor species containing the ions to be incorporated into the growing second layer of the nanoparticle. The fifth and sixth precursor species may be separate entities contained in said second semiconductor material precursor composition, and/or the fifth and sixth precursor species may be combined in a single entity contained in said second semiconductor material precursor composition.
The invention addresses a number of problems, which include the difficulty of producing high efficiency blue emitting dots.
The most researched and hence best-characterized semiconductor QD is CdSe, whose optical emission can be tuned across the visible region of the spectrum. Green and red CdSe/ZnS core-shell nanocrystals are the most widely available under existing methodologies. CdSe nanoparticles with blue emission along with narrow spectral widths and high luminescence quantum yields are difficult to synthesize using the conventional high temperature rapid injection “nucleation and growth” method. Using this conventional method to make blue quantum dots is difficult as the blue quantum dots are the smallest and are what is initially formed but rapidly grow (about 3 seconds of reaction time) in to larger does which have a green emission. There are also further problems including difficulties in experimental work-up, processes and overcoating with ZnS. Moreover, only small quantities of material can be produced in a single batch due to the dilute reaction solution necessary to keep the particle size small. Alternative blue emitting semiconductor nanocrystals include ZnTe and CdS, however, growing large (>4.5 nm diameter) ZnTe, needed for blue emissions, with narrow size distributions has proved difficult.
CdS on the other hand has an appropriate band gap and has been shown to emit in the 460-480 nm range with narrow size distributions and good luminescence efficiency. Bare CdS cores tend to emit white luminescence, attributed to deep trap emissions which can be suppressed by overcoating by a wide band gap material such as ZnS. These CdS/ZnS structures have shown recent promise as the active material for blue QD LED's and blue QD lasers.
Quantum Dots Incorporating Lower Toxicity Elements
Another drive for designing and producing specific quantum dot-quantum well structures in this invention is the current need for quantum dots free of elements (e.g. cadmium and mercury) which are deemed by national authorities to be toxic or potentially toxic but which have similar optical and/or electronic properties to those of CdSe—ZnS core-shell quantum dots. The current invention includes the design and synthesis of a number of cadmium free QD-QW structures based on II-VI/I-III-VI2/II-VI, III-V/II-V/III-V materials such as but not restricted to ZnS/CuInS2/ZnS, ZnS/CuGaS2/ZnS, ZnS/CuInSe2/ZnS, ZnS/CuGaSe2/ZnS.9,10,11,12 
Current Synthetic Methods
Many synthetic methods for the preparation of semiconductor nanoparticles have been reported, early routes applied conventional colloidal aqueous chemistry, with more recent methods involving the kinetically controlled precipitation of nanocrystallites, using organometallic compounds.
Over the past six years the important issues have concerned the synthesis of high quality semiconductor nanoparticles in terms of uniform shape, size distribution and quantum efficiencies. This has lead to a number of methods that can routinely produce semiconductor nanoparticles, with monodispersity of <5% with quantum yields >50%. Most of these methods are based on the original “nucleation and growth” method described by Murray, Norris and Bawendi, using organometallic precursors. Murray et al originally used organometallic solutions of metal-alkyls (R2M) M=Cd, Zn, Te; R=Me, Et and tri-n-octylphosphine sulfide/selenide (TOPS/Se) dissolved in tri-n-octylphosphine (TOP). These precursor solutions are injected into hot tri-n-octylphosphine oxide (TOPO) in the temperature range 120-400° C. depending on the size of the particles required and the material being produced. This produces TOPO coated/capped semiconductor nanoparticles of II-VI material. The size of the particles is controlled by the temperature, concentration of precursor used and length of time at which the synthesis is undertaken, with larger particles being obtained at higher temperatures, higher precursor concentrations and prolonged reaction times.
This organometallic route has advantages over other synthetic methods, including near monodispersity <5% and high particle cystallinity. As mentioned, many variations of this method have now appeared in the literature which routinely give high quality core and core-shell nanoparticles with monodispersity of <5% and quantum yield >50% (for core-shell particles of as-prepared solutions), with many methods displaying a high degree of size and shape control.1,2 
Recently attention has focused on the use of “greener”† precursors which are less exotic and less expensive but not necessary more environmentally friendly. Some of these new precursors include the oxides, CdO; carbonates MCO3 M=Cd, Zn; acetates M(CH3CO2) M=Cd, Zn and acetylacetanates [CH3COOCH═C(O−)CH3]2 M=Cd, Zn; amongst other.
†(The use of the term “greener” precursors in semiconductor particle synthesis has generally taken on the meaning of cheaper, readily available and easier to handle precursor starting materials, than the originally used organometallics which are volatile and air and moisture sensitive, and does not necessary mean that “greener precursors” are any more environmentally friendly).
Single-source precursors have also proved useful in the synthesis of semiconductor nanoparticle materials of II-VI, as well as other compound semiconductor nanoparticles. Bis(dialkyldithio-/diseleno-carbamato)cadmium(II)/zinc(II) compounds, M(E2CNR2)2 (M=Zn or Cd, E=S or Se and R=alkyl), have used a similar ‘one-pot’ synthetic procedure, which involved dissolving the precursor in tri-ii-octylphosphine (TOP) followed by rapid injection into hot tri-n-octylphosphine oxide/tri-n-octylphosphine (TOPO/TOP) above 200° C. Single-source precursors have also been used to produce I-III-VI2 materials i.e. CuInS2 using (PPH3)2CuIn(SEt)4 dissolved in a mixture of hexanethiol and dioctylphalate at 200° C. to give hexanethiol coated CuInS2.3 
I-III-VI2 nanoparticles have also been prepared from multi-source precursors such as in the case of CuInSe2 prepared from CuCl dissolved in triethylene and elemental indium and selenium. CuInTe2 was produce by a similar approach but from using elemental tellurium.
For all the above methods, rapid particle nucleation followed by slow particle growth is essential for a narrow particle size distribution. All these synthetic methods are based on the original organometallic “nucleation and growth” method by Murray et al, which involves the rapid injection of the precursors into a hot solution of a Lewis base coordinating solvent (capping agent) which may also contain one of the precursors. The addition of the cooler solution subsequently lowers the reaction temperature and assist particle growth but inhibits further nucleation. The temperature is then maintained for a period of time, with the size of the resulting particles depending on reaction time, temperature and ratio of capping agent to precursor used. The resulting solution is cooled followed by the addition of an excess of a polar solvent (methanol or ethanol or sometimes acetone) to produce a precipitate of the particles that can be isolated by filtration or centrifugation.
Preparation from single-source molecular clusters, Cooney and co-workers used the cluster [S4Cd10(SPh)16] [Me3NH]4 to produce nanoparticles of CdS via the oxidation of surface-capping SPh− ligands by iodine. This route followed the fragmentation of the majority of clusters into ions which were consumed by the remaining
Another method whereby it is possible to produce large volumes of quantum dots, eliminated the need for a high temperature nucleation step. Moreover, conversion of the precursor composition to the nanoparticles is affected in the presence of a molecular cluster compound. Each identical molecule of a cluster compound acts as a seed or nucleation point upon which nanoparticle growth can be initiated. In this way, nanoparticle nucleation is not necessary to initiate nanoparticle growth because suitable nucleation sites are already provided in the system by the molecular clusters. The molecules of the cluster compound act as a template to direct nanoparticle growth. By providing nucleation sites which are so much more well defined than the nucleation sites employed in previous work the nanoparticles formed in this way possess a significantly more well defined final structure than those obtained using previous methods. A significant advantage of this method is that it can be more easily scaled-up for use in industry than conventional methods.4 
The particular solvent used is usually at least partly dependent upon the nature of the reacting species, i.e. nanoparticle precursor and/or cluster compound, and/or the type of nanoparticles which are to be formed. Typical solvents include Lewis base type coordinating solvents, such as a phosphine (e.g. TOP), a phosphine oxide (e.g. TOPO) or an amine (e.g. HDA), hexanethiol, or non-coordinating organic solvents, e.g. alkanes and alkenes. If a non-coordinating solvent is used then it will usually be used in the presence of a further coordinating agent to act as a capping agent for the following reason.
If the nanoparticles are intended to function as quantum dots an outer capping agent (e.g. an organic layer) must be attached to stop particle agglomeration from occurring. A number of different coordinating solvents are known which can also act as capping or passivating agents, e.g. TOP, TOPO, alkylthiols or HDA. If a solvent is chosen which cannot act as a capping agent then any desirable capping agent can be added to the reaction mixture during nanoparticle growth. Such capping agents are typically Lewis bases but a wide range of other agents are available, such as oleic acid and organic polymers which form protective sheaths around the nanoparticles.